1. Field of the Invention
The present invention relates to a composition for substrate materials and process for the same, as well as a heat conductive substrate and process for the same, particularly to a technology for gaining a heat conductive substrate preferably utilized in the field of power electronics.
2. Description of the Related Art
Accompanying with the increased performance or miniaturization of electronic equipment, it is desired for semiconductor devices or electronic parts to be designed more densely or to be improved in performance. It is also desired for circuit substrates on which semiconductor devices or electronic components are mounted to be designed more densely and more miniaturized. Therefore it has become important to design a circuit substrate taking account of heat radiation. The following is an example of a heat conductive substrate which is a circuit substrate improved in heat radiating performance. This heat conductive substrate has a configuration where the wiring pattern is formed through an insulator layer on one or two sides of a metal plate such as copper or aluminum. In the following, this heat conductive substrate is referred to as a insulated metal substrate.
On the other hand, as an example of a heat conductive substrate improved in the heat radiating performance there is a heat conductive substrate in which a copper plate is integrated with a ceramic substrate formed of alumina, aluminum nitride or the like. In the following such a heat conductive substrate is referred to as a metal-attached ceramic substrate.
As for a metal-based substrate, it is preferable to make the insulator layer thinner to secure a better heat radiation. To make the insulator layer thinner, however, leads to disadvantages such that it becomes more susceptible to the effect of the noise or it becomes more difficult to secure enough withstand voltage against insulation.
And metal attached ceramic substrate can only be utilized for high current because of the reason that a manufacturing cost is higher compared to that of a metal-based substrate, therefore it is general to use a metal-based substrate for other objects.
In this way, it is difficult to achieve metal-based substrates and metal-attached ceramic substrates which satisfy both functions and manufacturing costs.
Therefore, in recent years, the following manufacturing process of heat conducting substrates is proposed. First of all, a resin composition where an inorganic filler with heat conductivity is filled up in a thermoplastic resin is prepared. And by injection molding and integrating this resin composition and a lead frame, a heat conductive module-type heat conductive substrate is formed.
Though a heat conductive module type heat conductive substrate manufactured in this manner can be secured in the mechanical strength better than that of metal-attached ceramic substrate, there occurs a disadvantage that heat radiation is worse because it is difficult to fill up inorganic filler in high density. The following is the reason why it is difficult to fill up inorganic filler in high density. Too much amount of filler increases the melting viscosity dramatically and makes it difficult to carry out kneading and injection molding. And because the filler works as an abrasive, the aberration of the metal mold is significant.
Therefore, recently, a heat conductive substrate manufactured by forming resin composition filled up with inorganic filler with good heat radiation integrated with a lead frame is proposed, as disclosed in, for example, the Japanese unexamined patent publication H10 (1998)-173097.
This heat conductive substrate is manufactured with the method shown in FIG. 9. That is to say, a mixture slurry including at least some inorganic filler and thermosetting resin is prepared. By forming a film from this mixture slurry a green sheet 31 is manufactured. After drying the green sheet 31, as shown in FIG. 9A, the green sheet 31 and the lead frame 32 are overlapped. After that, the green sheet 31 is cured by heating and pressurizing to produce a heat conductive substrate 34 constructed from a heat conductive cured body 33 and the lead frame 32 attached and integrated together, as shown in FIG. 9B.
In the above mentioned conventional heat conductive substrate 34, however, the following disadvantages occur. First, the green sheet 31 cannot be arranged without forming space with the lead frame 32 with the complicated form and the form precision cannot be fully maintained.
Moreover, in order to control the sheet thickness while manufacturing sheets, it is necessary to control and adjust the viscosity of the mixture slurry and a film-forming device, and work for that is difficult.
In addition, the sheet cutting process such as die punching using a metal mold for the convenience of processing sheets into a desired shape is indispensable, and therefore the processes have increased to raise the manufacturing costs.
In addition, the sheets cut off from the necessary parts cannot be utilized and they become dispensable, so it is not only disadvantageous from the point of cost effectiveness but also the resource is not utilized effectively.
Therefore, the main object of the present invention is to provide a heat conductive substrate with excellent heat radiation that is possible to be formed into a complicated form and the process for the same.
Another object of the present invention is to provide a process for a heat conductive substrate where control of mixture slurry or a film-forming device as well as sheet processing or the like are unnecessary.
Still another object of the present invention is to provide a composition for substrate materials configured such that a heat conductive substrate can be manufactured easily and economically as well as the manufacturing process for the same.
To achieve the above-mentioned objects, a composition for substrate materials of the present invention includes 70-95 wt. % of inorganic powder and 5-30 wt. % of thermosetting resin composition and is in a condition of crushed fine pieces. By using this composition for substrate materials, by curing the thermosetting resin on condition that inorganic powder is included in high density, a heat conductive substrate with good heat radiation including a high density of inorganic powder can easily be manufactured. Such a heat conductive substrate allows semi-conductor devices to be mounted directly thereon, and is extremely preferable as a heat conductive substrate used in the field of power electronics because it has a thermal expansion coefficient in the plane direction very close to that of semi-conductors.
And because the composition for substrate materials is in a condition of crushed fine pieces, it is possible for it to be filled with high precision even in a metal mold of complicated structure under the influence of the finely shaped lead frame or the like. In addition, it is possible to be filled in and be formed with the minimum amount of the composition for substrate materials to cause no waste of the composition for substrate materials. Moreover, such a formation with a high precision can be implemented under the conditions of low temperature and low pressure. The condition of finely crushed pieces is preferably a powder condition or a granulation condition.
It also preferable to coat the inorganic powder with thermosetting resin composition, thereby the adhesive properties between inorganic powder grains would be excellent.
It is also preferable for the thermosetting resin composition to be in a B-stage condition, thereby, as a result of less stickiness of the composition for substrate materials, it becomes easy to be handled.
It is also preferable for the inorganic powder to be at least one kind chosen among Al2O3, MgO, BN and AlN, thereby it gives the inorganic powder an excellent electric insulation and heat conductivity.
It is also preferable for the grain diameter of the inorganic powder to be within the range of 0.1-100 xcexcm, thereby being able to prevent the heat radiation from lowering. The larger grain diameters of the inorganic powder than this range would lower the density and therefore lowers the heat radiation. On the other hand, too small grain diameters would require to increase resin amount in order to fully contact powder grains with each other as a result of the increased surface area as a whole, which prevents the density from increasing and therefore lowers the heat radiation.
Moreover it is preferable that the thermosetting resin composition includes at least one kind of thermosetting resin chosen from among epoxy resin, phenol resin, cyanate resin as the main component, or that the thermosetting resin composition includes brominated polyfunctional epoxy resin as the main component, and, in addition, includes a bisphenol A-type novolac resin as a curing agent and imidazole as a curing accelerator.
In addition, it is preferable imidazole to be microencapsulated, thereby the storage stability for the composition for substrate materials would be improved because the curing accelerator doesn""t react with the thermosetting resin until the formation of the heat conductive substrate.
It is preferable to add to the thermosetting resin composition at least one kind chosen from among a coupling agent, a dispersing agent, a coloring agent and a mold-releasing agent.
According to a manufacturing process of the composition for substrate materials of the present invention, 70 to 95 wt. % of inorganic powder and 5 to 30 wt. % of thermosetting resin composition including thermosetting resin which is in a solid condition at a room temperature are crushed into fine pieces and mixed.
A manufacturing process of a composition for substrate materials according to the present invention includes a process of manufacturing a mixture by mixing 70 to 95 wt. % of inorganic powder and 5 to 30 wt. % of thermosetting resin composition including thermosetting resin which is in a liquid condition at a room temperature and a process for granulating said mixture.
A manufacturing of a composition for substrate materials according to the present invention has a process for mixing 70 to 95 wt. % of inorganic powder and 5 to 30 wt. % of thermosetting resin composition including thermosetting resin which is in a liquid condition at a room temperature and a process for turning said thermosetting resin into a B-stage by heat treating the mixture manufactured in the above process under temperature conditions lower than the curing temperature of said thermosetting resin and a process for crushing into fine pieces said mixture in which said heat setting resin composition is turned into a B-stage.
A manufacturing process of a composition for substrate materials according to the present invention has a process for mixing 70 to 95 wt. % of inorganic powder and 5 to 30 wt. % of thermosetting resin composition including thermosetting resin with solvent in which said thermosetting resin is dissolved and a process for drying the mixture manufactured in the above process at a lower temperature than the curing temperature of the said thermosetting resin to remove said solvent from said mixture and a process for crushing said mixture said solvent is removed from into fine pieces.
According to those manufacturing processes, a composition for substrate materials for manufacturing a heat conductive substrate can be extremely easily manufactured.
A heat conductive substrate according to the present invention has an insulation body formed, by heating and pressurizing, from a composition for substrate materials including 70 to 95 wt. % of inorganic powder having electric insulation and 5 to 30% of thermosetting resin composition and being in a condition of crushed fine pieces. This heat conductive substrate could secure good heat radiation while securing excellent mechanical strength.
It is also preferable for the wiring pattern to be provided on the condition that it is exposed on the surface of said insulator body. Said wiring pattern is preferably a lead frame. It is also preferable that a metal plate for heat radiation is provided on the condition that it is exposed on the surface of said insulator body.
A manufacturing process for heat conductive substrate according to the present invention has the first process where a composition for substrate materials including 70 to 95 wt. % of inorganic powder having electric insulation and 5 to 30% of thermosetting resin composition and being in a condition of crushed fine pieces is cast into a metal mold and the second process where said composition for substrate materials is formed into an insulator body by heating and pressurizing said composition for substrate materials within the metal mold to cure said thermosetting resin.
According to this manufacturing process, it is possible to manufacture a heat conductive substrate having complicated shape easily and with high precision without requiring control of slurry or a film-forming device, sheet processing or the like.
It is preferable that the pressure in which said composition for substrate materials are heated and pressurized is within the range of 1 to 20 MPa.
It is preferable that the temperature in which said composition for substrate materials are heated and pressurized is within the range of 140 to 260xc2x0 C.
It is preferable that, in said first process, said composition for substrate materials is cast into said metal mold while arranging a lead frame in the metal mold in advance and, in said second process, said insulator body and lead frame are formed and integrated by heating and pressurizing said composition for substrate materials. Or it is preferable that, in said first process, a lead frame is arranged in said metal mold in which said composition for substrate materials are cast and, in said second process, said insulator body and said lead frame are formed and integrated by heating and pressurizing said composition for substrate materials. By doing those processes, the composition for substrate materials goes into the gaps created by the lead frame having micro-structured and complicated form so that the insulator body and the lead frame are surely integrated. In addition, it is possible to form with minimum amount of composition for substrate materials filled in so that no waste results from the composition for substrate materials. In addition, because such a formation can be carried out under the conditions of low temperature and low pressure, not only is it economical but it also has the following advantages.
In an injection mold or the like which is a general forming method, extremely large injection pressure is applied so that the resin should be spread into every corner of the metal mold. Therefore, in case a heat conductive substrate with a lead frame is formed by injection mold or the like, the resin reaches onto the surface of the lead frame, which makes it necessary to set up a process for removing this extra resin after the forming process.
Contrary to that, because the composition for the substrate materials has a form of finely crushed pieces, the composition for substrate materials can be spread into every corner including the gaps with the lead frame by applying little pressure on casting the composition for substrate materials into the metal mold. Therefore, extra resin would not go onto the surface of the lead frame exposing the insulator body. Accordingly it is not necessary to have a process for removing such extra resin.
To resolve the disadvantages of the above mentioned injection mold, there is a method to integrate the green sheet described in the prior art and the lead frame. In this method, however, the green sheet cannot be fitted into the parts with complicated forms and the following disadvantages are included.
In a heat conductive substrate, it is desired to have a thick lead frame because of the reason that heat radiation should be enhanced or high current should be applied. In the method using the green sheet, however, the formation precision cannot be maintained when the lead frame is thick because the heat conductive substrate is manufactured by pushing the green sheet into the gap with the lead frame.
On the contrary, according to the present invention, the composition for substrate materials can be spread into every corner in the gaps created by a thick lead frame because finely crushed composition for substrate materials is cast into a metal mold so that it can be applied for a thick lead frame.
It is preferable that, in said first process, a metal plate for heat radiation is arranged within said metal mold in which said composition for substrate materials is cast and, in said second process, said composition for substrate materials is heated and pressurized to mold integrally said insulator body and said metal plate. Or it is preferable that, in said first process, a metal plate for heat radiation is arranged in advance within the metal mold and then said composition for substrate materials is cast into the metal mold and, in said second process, by heating and pressurizing said composition for substrate materials, said insulator body and said metal plate are molded integrally. In this manner, the metal plate for heat radiation is integrated into the insulator body.
Moreover, in the first process, a metal foil is arranged in said metal mold in advance then said composition for substrate materials is cast into the metal mold and, in said second process, by heating and pressurizing said composition for substrate materials, the insulation body and said metal foil are molded integrally and then said metal foil is pattern-processed into a wiring pattern. Or it is preferable that, in said first process, a metal foil may be arranged in the metal mold after the composition for substrate materials is cast therein and in said second process, by heating and pressurizing said composition for substrate materials said heat conductive substrate and said metal foil are molded integrally and then said metal foil may be pattern-processed into a wiring pattern.
It is preferable that said metal foil is copper foils with a thickness of 12 to 200 xcexcm and is roughened at least on the surface facing the composition for substrate materials. Thereby, the adhesive strength with the insulator body would be enhanced.
Here a room temperature is generally specified to be a temperature range between about xe2x88x9210 to 40xc2x0 C.